Ink-jet printing has become an established printing technique and generally involves the controlled delivery of ink drops from an ink containment structure, or reservoir, to a printing surface.
One type of ink-jet printing, known as drop-on-demand printing, employs a pen that has a print head that is responsive to control signals for ejecting drops of ink from the ink reservoir. Drop-on-demand ink-jet printers typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave. The print head of a thermal bubble type pen includes a thin film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice.
Piezoelectric pressure wave systems use a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of ink in the print head to thereby produce a pressure wave that forces the ink drops through the orifice.
Although conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, they do not include any mechanism for preventing ink from permeating through the print head when the print head is inactive. Accordingly, drop-on-demand techniques require a slight back pressure at the print head to prevent ink from leaking through an inactive print head.
One prior technique for providing sufficient back pressure at the print head is described in U.S. Pat. No. 4,771,295, issued to Baker et al. The system in Baker et al. employs a porous synthetic foam within the ink reservoir. The capillarity of the foam provides the back pressure necessary for preventing the ink from permeating through the print head whenever the print head is inactive.
One problem associated with the use of foam for establishing back pressure at the print head is that some of the ink in the reservoir will become trapped in the very small pores of the foam. Specifically, pore size in foam varies throughout the volume of the foam. The very small pores in the foam exert on the ink a correspondingly strong capillarity that cannot be overcome by the pumping effect of a conventional print head. Any amount of ink that remains trapped in the pen reduces the volumetric efficiency of the pen, which efficiency can be quantified as the interior volume of the pen divided by the total volume of the ink that is delivered by the print head.